The present invention relates to a frequency analyzing device and, more particularly, to a frequency analyzing device for analyzing the frequency of an input signal using various orthogonal function system signals.
Generally, when the frequency components of an input signal which continuously changes over time are to be analyzed using various orthogonal function system signals, a frequency analyzing device (or a frequency spectrum measuring device) called a spectrum analyzer is used.
There are two basic measurement processing methods used by frequency analyzing devices of this type.
As one method, all arithmetic processing operations for an input signal are performed as analog processing on the basis of Fourier transform formulas of a continuous wave.
Fourier transform formulas for an input signal f(t) that continuously changes over time are represented by equations (1) to (3):                               f          ⁡                      (            t            )                          =                                            a              0                        2                    +                                    ∑                              n                =                1                            ∞                        ⁢                          xe2x80x83                        ⁢                          (                                                                    a                    n                                    ⁢                                      cos                    ⁡                                          (                      t                      )                                                                      +                                                      b                    n                                    ⁢                                      sin                    ⁡                                          (                      t                      )                                                                                  )                                                          (        1        )                                          a          n                =                              1            π                    ⁢                                    ∫              0                              2                ⁢                π                                      ⁢                                          f                ⁡                                  (                  t                  )                                            ⁢                              cos                ⁡                                  (                                      n                    ⁢                                          xe2x80x83                                        ⁢                    t                                    )                                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                                                        (        2        )                                          b          n                =                              1            π                    ⁢                                    ∫              0                              2                ⁢                π                                      ⁢                                          f                ⁡                                  (                  t                  )                                            ⁢                              sin                ⁡                                  (                                      n                    ⁢                                          xe2x80x83                                        ⁢                    t                                    )                                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                                                        (        3        )            
where an and bn are Fourier coefficients and represent a cosine wave component and a sine wave component, respectively. The magnitude, e.g., power spectrum P(n) of a predetermined analysis frequency component contained in the input signal f(t) can be obtained from an and bn on the basis of equation (4):
P(n)={square root over ((an+L 2+L +bn+L 2+L ))}xe2x80x83xe2x80x83(4)
When the input signal f(t) that continuously changes over time is multiplied by sine and cosine wave signals having an analytic frequency of interest, and the results are integrated at one period (0 to 2xcfx80) of the input signal f(t), as in equations (1) to (3), the cosine and sine wave components of the analytic frequency contained in the input signal f(t) can be obtained.
FIG. 16 shows the arrangement of a conventional frequency analyzing device based on analog processing. The frequency analyzing device comprises a sine wave generator 93, analog multiplier 91, and analog integrator 94.
In this case, a sine wave output 92 generated by the sine wave generator 93 is multiplied by an input signal f(t) 96 input from an external device and continuously changing over time by the analog multiplier 91. The output is integrated at several periods by the analog integrator 94, and a desired frequency component is detected.
As the other method, after an input signal is converted into a digital signal, subsequent processing is performed by calculating numerical values using discrete Fourier transform formulas.
In this method, an input signal is sampled (sampling and digital conversion) by an A/D converter, and subsequent processing is performed by calculating numerical values by discrete Fourier transform processing using a DSP (Digital Signal Processor). With this method, spectra can be calculated from the input signal in units of different analytic frequencies.
However, in such a conventional frequency analyzing device, e.g., the frequency analyzing device based on analog processing, the circuit scale of the sine wave generator 93 and analog integrator 94 becomes large, and the circuit cannot be integrated on a small semiconductor chip.
For example, to generate an analog sine wave, a capacitor or inductor with a relatively large capacitance or inductance is required depending on its frequency.
The analog integrator 94 is realized by an operational amplifier and capacitor. To realize this using a semiconductor integrated circuit, a number of elements are necessary, and the chip area increases. In addition, to realize a capacitor with a large capacity. using a semiconductor integrated circuit, the chip area further increases.
When spectra of a plurality of frequencies are to be simultaneously measured, the above-described frequency analyzing devices equal in number to desired frequencies must be prepared. In this case, the circuit scale further increases.
In this case, in the above-described conventional frequency analyzing device (FIG. 16), one analog multiplier and one analog integrator may be used, and the frequency output 92 from the sine wave generator 93 may be swept to measure a plurality of frequency spectra from the input signal 96.
In this case, however, frequency sweeping takes time, a plurality of frequency components cannot be simultaneously measured, and only a frequency component of the input signal, which has a repetitive waveform, can be measured.
According to the frequency analyzing device based on digital processing, to execute frequency analysis at a high speed equal to that of the frequency analyzing device based on analog processing, a high-speed A/D converter and DSP, and for example, a memory accessible at a high speed and product-sum operation circuit are required. For this reason, the circuit scale becomes large, and power consumption increases.
To speed up numerical calculation, an algorithm called FFT (Fast Fourier Transform) is improved and widely used. When the number of samples of an input signal is N, product-sum operation of complex numbers need be performed 2NlogN times.
Hence, even when a spectrum is to be calculated at a high speed using a digital calculation circuit, the spectrum cannot be obtained immediately after measurement.
The present invention has been made to solve the above problems, and as its object to provide a frequency analyzing device which can reduce the circuit scale and can be integrated on a small semiconductor chip without decreasing the speed of frequency analysis.
In order to achieve the above object, according to the present invention, there is provided a frequency analyzing device for detecting a signal component of a predetermined analytic frequency to be analyzed in an input signal, comprising a multiplication circuit which controls a switched capacitor circuit for discretely storing an input signal using a predetermined pulse signal whose frequency changes according to an amplitude of an orthogonal function system signal having the analytic frequency, and outputs charges representing a multiplication result of the input signal and the orthogonal function system signal, and an integration circuit for integrating the charges output from the multiplication circuit and outputting integration values as signal components of the analytic frequency contained in the input signal.